In the manufacture of vehicles, body shells which are conveyed from a body shell construction line to an enameling line painting station generally have surface defects which are regarded by a customer as quality- lowering defects unless subjected to finishing operations. Consequently, such surface defects in body shells are to be detected and reworked.
In the past, relevant surface defects in series production have been recognized and evaluated subjectively by an inspector, especially by probing while wearing gloves, by display from different viewing directions and, if desired, by stripping with a whetstone if the inspector's experience has been that surface defects often occur at specific points on a body shell type.
Surface defects such as this may be dents/bulges, collapses, injection point/solder voids, uneven areas/hairline cracks, etc. The geometric parameters in evaluation are depth, extent, gradients, surface curvature, local frequencies of undulations, and positions of surface defects. Determination of whether a surface defect is relevant for a reworking operation normally depends on evaluation of a combination of the parameters indicated. For example, a dent 20 .mu.m deep and 50 mm long can easily be recognized and must be reworked, while a dent of the same depth and 200 mm long normally is not recognized as a surface defect impairing quality and accordingly no reworking is required. Such combinations and relationships of parameters are of necessity made by an inspector in subjective evaluation, no clear-cut boundaries existing between the various relevant surface defects present resulting from such combinations.
In view of the complex and variable defect patterns indicated, automated computer recognition of such relevant surface defects has not been possible in the past.
In addition, the surface of body shells from body shell operations is often coated with a film of oil which is dull, diffused and dirty. This situation makes both subjective detection of surface defects by an inspector and automated recognition difficult. Relevant surface defects not recognized in the body shell are not recognized until after base enameling, when additional inspection is required. It is apparent that the cost of reworking surface defects is the higher the farther the body has advanced in the manufacturing process, and especially in the enameling process.
In order to reduce the high cost of recognizing relevant surface defects and of regular late reworking after base enameling, a number of attempts have already been made and experiments conducted to automate recognition of surface defects requiring reworking by measurement systems and machines, with the aim of recognizing as large a number of relevant surface defects as possible before enameling.
State-of-the-art systems, for example, involve operation by a strip projection process or on the basis of moire interferometry, in which adequate measurement accuracies of about 10 .mu.m can be achieved by means of phase-shift processes. These processes do, however, require a time of approximately 1 minute for recognition and evaluation of an area measuring approximately 250 mm.times.250 mm. Such measurement processes are accordingly much too long for series production, in which a normal conveyor belt speed for body shells is about 4 meters per minute.
An alternative state-of-the-art process, called retroreflex process, has also been tested for recognition of surface defects in body shells. In this process the surface of a body shell is irradiated with a light beam. The radiation is reflected at the angle of incidence of a retroflector which is positioned perpendicularly to the surface examined. Display of surface defects is markedly intensified by multiple reflection of the light beam and so can be recognized by means of relatively simple image processing operations. A disadvantage of this process is, of course, that it is feasible only if a brightening agent is used. This brightening agent is to be applied to the surface before detection and recognition and rinsed off again after recognition. This represents a costly additional process step involving expenditure for the brightening agent. In addition, the brightening agent is a chemical harmful to the environment which creates the problem of disposal. Consequently, this process as well is to be rejected for use in series production.
The object of the invention is creation of a process for automatic recognition of surface defects requiring reworking, one suitable for series production, and of a device for application of this process.